Fundus camera imaging is acknowledged to be an important diagnostic tool for detection of various conditions affecting the eye, including diabetic retinopathy and macular degeneration. Various embodiments of fundus imaging apparatus are disclosed, for example in U.S. Pat. No. 5,713,047 (Kohayakawa); U.S. Pat. No. 5,943,116 (Zeimer); U.S. Pat. No. 5,572,266 (Ohtsuka); U.S. Pat. No. 4,838,680 (Nunokawa); U.S. Pat. No. 6,546,198 (Ohtsuka); U.S. Pat. No. 6,636,696 (Saito); U.S. Pat. No. 4,247,176 (Ito); U.S. Pat. No. 5,742,374 (Nanjo et al.); and U.S. Pat. No. 6,296,358 (Cornsweet et al).
While these patents attest to continuous improvements in fundus camera design, there are still significant hurdles to obtaining good quality images from these devices. Fundus cameras must solve the fairly difficult problem of simultaneously illuminating the retina through the pupil and obtaining the retinal image, with both illumination and image-bearing light traveling along substantially the same optical path. One particularly troublesome problem relates to stray light caused by unwanted reflection from the lens surface of the patient's eye itself as well as from optical surfaces within the camera apparatus. Unless its level is controlled, this unwanted reflected light can degrade image contrast and overall image quality.
This problem is most readily illustrated by an overview of the operation of the illumination subsystem in a conventional fundus imaging apparatus. Referring to FIG. 1, there is shown a fundus imaging apparatus 10 in which a conventional illumination section 12 is used. The patient's eye E is positioned along an optical axis O using an alignment subsystem (not shown in FIG. 1). Illumination section 12 directs light either from an observation light source 14 and a lens 16 or from an image capture light source 18 and a lens 20 as controlled by control logic circuitry for fundus imaging apparatus 10 (not shown in FIG. 1). A dichroic mirror 22 directs light from the appropriate source through a ring-slit diaphragm 24 and a lens 26, to an apertured mirror 28. Apertured mirror 28 directs the illumination light along axis O and through an objective lens 42 toward the pupil for illuminating the retina of eye E. Depending on the use of fundus imaging apparatus 10 at any one time, either observation light source 14 or image capture light source 18 are activated. Observation light source 14 is typically infrared (IR) light, to which eye E is insensitive. Image capture light source 18, on the other hand, may be a high-brightness source such as a xenon lamp, for example. Depending on the application, image capture light source 18 may be pulsed or strobed.
Ring-slit diaphragm 24 has the characteristic functional arrangement shown in FIG. 2. Light is transmitted through an inner ring 30 and is blocked at a middle section 32 and at an outer section 34. As is shown in the received illumination ring of FIG. 3, inner ring 30 is directed into a pupil 36 of the patient as a ring 40 of illumination. To obtain the retinal image, apertured mirror 28 (FIG. 1) has an aperture suitably centered about optical axis O to allow light that has been reflected from the retina of eye E and directed through lenses 42 and 44 to reach a sensor 46, such as a CCD.
The high-level block diagram of FIG. 1 thus gives an overview of illumination section 12 that applies for conventional fundus imaging apparatus. There have been numerous methods disclosed for optimizing the performance of illumination section 12, including components arranged to prevent stray reflected light from the cornea of eye E and from optical surfaces from being directed back toward sensor 46. Referring to the schematic block diagram of FIG. 1, three basic approaches have been followed in order to reduce or eliminate stray light from these sources:                (i) Using a pair of crossed polarizers. Using this approach, a first polarizer 600 is placed in the illumination path, prior to apertured mirror 28. A second polarizer 602 is then positioned in the image path, following apertured mirror 28. With reference to FIG. 1, first polarizer 600 and second polarizer 602 are positioned as shown at phantom locations. The polarizers 600 and 602 are cross-aligned so that the light reflected back from the lens surfaces can be blocked by polarizer 602.        There are two key problems with this method. The first problem relates to the needed high power lamp when using this strategy. Because only that portion of light having the proper polarization is transmitted through polarizer 600, more light is needed from image capture light source 18. The use of second polarizer 602 further blocks the useful light reflected from the retina by 50%. As a result, the power of light source 18 must be about 4 times as high as would be necessary without polarizers 600 and 602. The second problem relates to the nature of light reflected from the cornea. Since this light can be depolarized, particularly due to the large incident angle, second polarizer 602 will be less effective in blocking unwanted stray light.        (ii) In the illumination path, blocking reflected light which, otherwise, will reflect from the lens surface and reach the sensor 46. This solution, however, reduces uniformity of the desired light reflected from the retina, particularly noticeable when attempting to obtain retinal images from near-sighted patients.        (iii) Separating illumination and imaging optical paths. A beamsplitter can be placed in front of objective lens 42 to effect this separation. However, this type of solution requires additional light power in order to obtain suitable reflected light from the retina and necessitates a longer working distance for objective lens 42.        
Reflective optics have been used in display apparatus that require a highly compact optical arrangement. For example, U.S. Pat. No. 5,889,625 (Chen et al.) discloses an optical arrangement that directs an image-bearing light to a human observer using a curved mirror as part of a head-mounted device (HMD). Similarly, U.S. Pat. No. 5,499,139 (Chen et al.) discloses a helmet-mounted optical apparatus for providing a wide-field image to a pilot, where the optical apparatus employs a curved mirror and compensation for image aberration. However, while mirrors have been used effectively in display applications of this type, the use of curved mirrors in a system that must simultaneously illuminate and capture an image is understandably much more difficult.
Mirrors have also been utilized in some more complex ophthalmological cameras for imaging internal structures of the eye. For example, U.S. Pat. No. 5,847,805 (Kohayakawa et al.) discloses an apparatus for scanning a pair of beams into the eye using a combination of rotary polygon scanning mirror and a galvanometric mirror. Similarly, U.S. Pat. No. 6,585,374 (Matsumoto) discloses various embodiments using a movable concave mirror mounted on a rotation axis for imaging different portions of the eye from different rotated positions. The apparatus of U.S. Pat. Nos. 5,847,805 and 6,585,374 are relatively costly, high-end ophthalmological imaging devices that require added movable components in the optical path in order to obtain multiple images for diagnosis.
There is a need for inexpensive fundus imaging cameras where scanning operation is not needed. This less complex type of camera is designed for use in physician's offices and is used for first-level screening for diabetic retinopathy, for example. With such an apparatus, a single retinal image from each eye is all that is needed for screening.
In summary, there is a need for a lower cost optical system in a fundus imaging camera that reduces stray light from lens surface reflection without significantly increasing the needed illumination brightness and without adversely affecting image quality.